The purpose of this work is to study the factors regulating and controlling the sudden switch from 2,3 bisphophoglycerate (2,3-DPG) as the major organic phosphate of the chick embryo erythrocyte just before hatch to inositol pentaphosphate (IPP) as the major organic phosphate just after hatching. We have isolated and purifed the enzyme 2,3-bisphosphoglycerate phosphatase from red blood cells of the day-old chick. The elution profiles for bisphosphoglycerate phosphatase and 2,3-bisphosphoglycerate mutase were identified upon gradient elution from columns of hydroxylaptite and diethylaminoethylcellulose and by gel filtration on Sephadex G-100. At each stage of purification the ratio of phosphatase to mutase activity was the same. The enzyme activity in erythrocytes decreases gradually from its maximum in the two-day chick (1.67 micron mol/h/g hemoglobin) and by 27 days it has fallen nearly to the levels seen in the adult chicken (0.11 micron mol/h/g hemoglobin) The purified bisphosphoglycerate phosphatase-mutase protein also contained about 6% of the cell 3-phosphoglycerate mutase activity which has now also been purified. We have concluded that, just as in the human erythrocyte, both 2,3-DPG phosphatase and 2,3-DPGmutase activities in the chick erythrocytes residue on one protein. This system is therefore very similar to that found in man.